Ball valves are among other things used for modulating control of fluids in HVAC and industrial process applications.
Full ported ball valves have a very large flow coefficient (CV-value) compared to its size. This means that if the valve is connected to a pipe of the same size and the flow rate is within the normally specified range for the pipe size, the pressure drop produced by the ball valve will be very small, when fully open.
The pipe and the controlled object, such as a heat exchanger, coil etc, have much larger pressure drops. This means that the ball valve has very little authority over the flow as it begins to close. The valve needs to close so much that it produces a pressure drop almost as large as the control object and pipe combined, before any significant reduction of the flow rate will take place.
This means that a large portion of the operating range of the valve is ineffectual. Control of the flow will only take place over a very small portion of the operating range, near the closed position. Therefore, it is hard for the actuator to operate the valve to the exact position that supplies the correct flow rate. Even a small movement causes a disproportionately large change in the output from the connected heat transfer device, and control stability is hard to achieve.
The flow coefficient is calculated by multiplying the FLOW (GPM) by the square root of the specific gravity of the fluid and then divide by the square root of the differential pressure (PSI) across the fully open valve.
Valve sizing is based upon the flow coefficient. A valve must have a sufficiently large flow coefficient so the needed maximum flow rate can be supplied. However, it must not be too large because it causes control problems.
The very large flow coefficient of a typical ball valve can be reduced by installing a characterizing disk with an essentially V-shaped opening, that interacts with the bore of the ball in the valve. This also determines the flow characteristics of the valve. U.S. Pat. No. 6,039,304 describes such a disk.
By changing the disk to a new disk with a smaller or larger opening, the flow coefficient can be changed, without replacing the valve.
The disk works very well, but it adds to the cost of the valve, and if the flow coefficient needs to be changed a new disk needs to be ordered and installed. Especially disks used in high temperature and high pressure process applications are relatively expensive.
As an alternative it is possible to use a ball with a specially shaped opening. When the flow coefficient needs to be changed, a new ball with a differently shaped opening is installed. However, this is costly alternative.
It is common that the flow coefficient needs to be changed, because the data the flow coefficient calculations are based upon often are very unreliable and does not reflect the actual operating conditions.
Even if the installed flow coefficient is correct when first installed, future changes in the operation conditions may necessitate a change of the flow coefficient.